1. Field of the Invention
The present invention relates to a reticle management system, and in particular to a reticle management system which includes a reticle sorter capable of operating with a plurality of modular bare reticle stockers and/or closed container stockers.
2. Description of Related Art
Semiconductor devices on wafers are made up of as many as fifty individual patterned layers of silicon, silicon compounds and metals. During fabrication of these devices, the pattern for each of these layers is contained on a mask called a reticle. A reticle is an optically clear quartz substrate on which a pattern has been formed by photolithography or other such processes. In particular, a layer of photoresist is applied on a chrome coated reticle blank. Thereafter, the pattern for a particular layer to be formed on a semiconductor wafer is transferred onto the reticle as for example by a laser pattern generator or e-beam. After pattern generation on the photoresist, the exposed portions of the photoresist are removed to leave the unwanted portions of the chrome layer exposed. These unwanted portions are then etched away. The remaining photoresist is then removed in a process which leaves the clean pattern on the surface of the reticle.
In order to keep the surface of the reticle clean, a thin transparent sheet called a pellicle is mounted a short distance away from the surface of the reticle containing the pattern. This ensures that any microscopic dust that settles on the reticle will be out of focus during the exposure process so as not to affect the pattern formed on the silicon wafer.
During fabrication of the reticle, it is important to minimize airborne particle fluxes onto the surface of the reticle on which the pattern is being formed, as any such particles can corrupt the pattern. Even after formation of the pattern and affixation of the pellicle, larger, or macro, contaminants can settle on the reticle which can interfere with pattern transference onto the semiconductor wafer. It is therefore advantageous to prevent particles from adhering to reticles during reticle fabrication, during storage of the reticles, during transfer of the reticle from the reticle fab to the semiconductor fab and during usage of the reticle in the semiconductor fab.
During semiconductor and reticle fabrication, it is known to store such workpieces within large storage units referred to as stockers. A conventional stocker for storing bare reticles is typically a large unit having a plurality of shelves on which the reticles may be stored, and a transport system for transferring reticles into and out of the stocker and for moving reticles around within the stocker.
Conventional reticle stockers can typically hold as many as 10,000 bare reticles. These units take up a considerable amount of space within a reticle or wafer fab, where space is at a premium owing to the requirement of clean room conditions. Moreover, there is currently no provision for customizing a stocker to given storage and space requirements of a particular reticle or wafer fab.
It is another consideration in a wafer fab whether to store workpieces such as reticles as individual, bare substrates or whether to store them within closed containers. Some semiconductor fabs are located in areas prone to seismic activity and/or have unreliable power supply. In the event of an earthquake or power outage (so that the fan/filter unit stops operating), there is less likelihood of damage to the workpieces if they are stored in closed containers. On the other hand, some semiconductor fabs are located in areas that have low seismic activity and reliable power supply, and protection of the workpieces against earthquakes and/or power outages is not as critical. These areas may choose to store bare reticles, with no container, as such storage systems generally take up less footprint in the clean room environment of the fab where space is always critical. At present, no single system exists which may be configured as desired by a particular manufacturer to address its particular workpiece storage needs.